1. Field of the Invention
The present utility model relates to a sheet take-up device for a projection screen and the like which can freely set the support position of a sheet such as a projection screen and the like drawn out, can take up the sheet by only drawing down slightly and then releasing the sheet, can shorten the length of drawing out for this drawing down, and can prevent the operation of drawing out or taking up the sheet from being interrupted on the way in spite of its simple structure.
2. Description of the Related Art
There is a demand on a sheet take-up device for a projection screen and the like that it is possible to freely set the support position of a sheet drawn out and shorten the length of drawing out the sheet when taking up the sheet, and in order to cope with this demand various mechanisms have been proposed up to now and for example, Japanese Utility Model Publication No. Hei 7-2533 is also one of them.
This sheet take-up device is provided with a guide drum outside a support shaft and a drum for taking up a sheet outside this guide drum, supports the drum turnably on the support shaft by energizing the drum in the sheet take-up direction, supports the guide drum on the support shaft so as to be turnable only in the sheet draw-out direction by interposing a one-way clutch between the support shaft and the guide drum, provides an endless cam groove on the outer circumferential face of said guide drum, and interposes a changeover ball engaged with the cam groove between the drum and the guide drum so as to be freely move in the axial direction of the guide drum.
In this sheet take-up device, when the sheet is drawn out the drum is turned in the sheet draw-out direction, and when the drawing of the sheet is cancelled the drum is turned in the sheet take-up direction and the changeover ball is moved to engage with the fixing engagement portion of the cam groove and the one-way clutch operates to regulate turning of the drum in the sheet take-up direction and the sheet is supported in a hanging state.
And in order to take up the sheet drawn out, it is enough to slightly draw out the sheet and cancel the drawing of the sheet at the point of time when the changeover ball is engaged with the temporary engagement portion of the cam groove, and the changeover ball is moved to the return groove of the cam groove by a turning force energized in the drum, and the engagement of the drum with the guide drum is released and the drum is turned in the sheet take-up direction and thereby the sheet is taken up.
By the way, since such a sheet take-up device as described above adopts a one-way clutch to stop and hold the drum when the sheet is supported in a hanging state, it needs a ratchet wheel, a ratchet pawl and the like, and has a problem that the device is made high in manufacturing cost due to not only the increase in number of components but also the complication in structure of it.
And the structure in which a guide drum having a cam groove is fixedly arranged and a changeover ball is moved in the axial direction of the guide drum has a problem that resistance occurs when the changeover ball is moved and it is difficult to smoothly draw out and take up the sheet.
Thereupon, an object of this utility model is to provide a sheet take-up device for a projection screen and the like which makes it possible to omit a one-way clutch, simplify said device in structure by reducing components in number, and smoothly draw out and take up a sheet.
In order to solve such a problem as described above, the present utility model provides a sheet take-up device for a projection screen and the like, comprising a cam spline which is attached to a fixed support shaft so as to be unable to turn and able to move in the axial direction and has an endless cam groove provided on its outer circumferential face, a cam holder supported on the outer circumferential face of this cam spline so as to be able to turn, a sheet take-up drum fixed to the outer circumferential face of the cam holder so as to turn as one body with said cam holder, a changeover ball held by said cam holder at a fixed position so as to engage with said cam groove, and a take-up elastic force providing means which is provided between said support shaft and said take-up drum and accumulates the elastic energy in the sheet take-up direction for the take-up drum by the rotation in the sheet draw-out direction of the take-up drum, wherein said endless cam groove provided on the outer circumferential face of said cam spline is formed out of a draw-out groove and a take-up groove which are spaced in the axial direction and ring-shaped in the circumferential direction, and an engagement groove and a return groove which make said draw-out groove and said take-up groove communicate with each other, said engagement groove is provided with a fixing engagement portion for making the cam spline and the changeover ball engage with each other so as not to turn the take-up drum in the sheet take-up direction and a changeover engagement portion for releasing said engaged state by turning of the changeover ball in the sheet take-up direction, said fixing engagement portion and changeover engagement portion being curved in directions opposite to each other with respect to the circumferential direction of the cam spline, said take-up groove moves the cam spline so that the return groove advances to the changeover ball by turning of the changeover ball in the sheet draw-out direction, said return groove moves the cam spline so that the draw-out groove advances to the changeover ball by turning of the changeover ball in the sheet draw-out direction, and said draw-out groove moves the cam spline so that the engagement groove advances to the changeover ball by turning of the changeover ball in the sheet draw-out direction.
Here, a structure can be adopted in which said cam spline is fitted onto the outer circumference of a spline shaft fitted and fixed onto the support shaft so as to be unable to turn and is coupled with the spline shaft so as to able to freely move in the axial direction and unable to turn relative to the spline shaft by interposing balls between spline grooves respectively provided in the inner circumferential face of the cam spline and in the outer circumferential face of the spline shaft.
And a structure may be adopted in which a speed regulating mechanism for regulating the turning speed in the sheet take-up direction of the take-up drum so that the turning speed does not become too high is provided between said support shaft and said take-up drum, and this speed regulating mechanism is formed using an oil rotation damper.